Heretofore, exhaust gas recirculation (EGR) has been widely implemented in which an EGR passage is provided to communicate between an exhaust passage and an intake passage of an engine, and allow a part of exhaust gas flowing through the exhaust passage to be recirculated to the intake passage therethrough. The EGR results in increased ratio of an inert gas component (recirculated exhaust gas, i.e., EGR gas) to an intake air, so that it becomes possible to lower combustion temperature, thereby suppressing formation and emission of nitrogen oxides (NOx). In addition, a total intake gas amount can be increased while suppressing an increase in amount of oxygen, so that it becomes possible to reduce an intake negative pressure and thereby reduce pumping loss, while suppressing formation and emission of NOx.
A turbocharger has also been widely known which is designed to achieve a high engine power output by utilizing exhaust energy. The turbocharger is a mechanism in which a turbine wheel (hereinafter occasionally abbreviated as “turbine”) provided in an exhaust passage and a compressor wheel (hereinafter occasionally abbreviated as “compressor”) provided in an intake passage are coupled together through a coupling shaft, in such a manner that, when the turbine is rotated by a pressure of exhaust gas, the compressor is driven to compress intake air, thereby causing a rise in air charging pressure.
JP 2000-161131A discloses an engine in which an EGR passage is formed in a cylinder head, and an inlet of the EGR passage is opened to an exhaust collecting portion formed by collecting together two or more exhaust ports each extending from a respective one of a plurality of combustion chambers (paragraphs [0033] and [0034]).
JP 2009-114991A discloses an engine in which an inlet of an EGR passage is opened to a region of an exhaust passage upstream of a turbine of a turbocharger (paragraph [0061]).
Meanwhile, in the case where an inlet of an EGR passage is opened to a region of an exhaust passage upstream of a turbine of a turbocharger, as disclosed in JP 2009-114991A, a passage volume upstream of the turbine is increased, so that exhaust gas is expanded, resulting in lowering of pressure upstream of the turbine. As a result, there is a problem of lowering in turbine driving force (driving force for the turbine).